In a single wafer rapid thermal processing (RTP) reactor, one of the critical process parameters is the temperature of the wafer. The heat needed to drive the process in RTP is provided by a heating lamp module which consists of high-intensity lamps (usually tungsten-halogen lamps or arc lamps) and a properly designed reflector for directing radiated photons onto the wafer. The lamp reflector is designed to provide a uniform temperature distribution across the surface of the wafer. Advanced VLSI fabrication processes demand a tight control over the temperature distribution of the wafer during the process. Usually the wafer temperature gradients should be kept below .+-.5.degree. C. (preferably less than .+-.2.degree. C.) in order to drive the desired fabrication process uniformly over the entire surface of the wafer and in order to minimize any defect formation on the wafer by thermally induced stresses.
However, lateral temperature gradients are common problems in lamp-heated RTP of semiconductor materials. The wafer temperature nonuniformities are direct results of the RTP hardware and process requirements which are related to the capabilities for fast wafer heating and cooling. For fast control over temperature and efficient heating of the wafer, the thermal mass of the wafer and a holder for the wafer within the RTP must be minimized. Low thermal mass usually requires operating the system in a cold-wall mode.. Therefore, in contrast to the conventional hot-wall furnace processing where the entire volume of the furnace is heated isothermally, the RTP systems usually employ lamps to selectively heat the wafer using either the free-carrier absorption or through a direct band-to-band photon absorption mechanism. Fast wafer cooling can be easily accomplished in a cold-wall RTP reader where the process chamber walls are not heated during the high-temperature process. All other wafer heat loss mechanisms, except for radiative losses, are also avoided. Lamp heating of the wafer minimizes the thermal mass effects of the process chamber and insures a rapid real-time control over the wafer temperature. However, since the wafer does not sit on a heating chuck (which would provide uniform wafer heating through high thermal mass and high thermal conductivity), the temperature uniformity becomes sensitive to the uniformity of the optical energy absorption as well as the radiative and convective heat losses (and residual conductive heat losses, if any) of the wafer.
The wafer temperature nonuniformity usually manifests itself in the form of radial temperature gradients with most of the nonuniformity localized near the wafer edge. This kind of nonuniformity typically has a cylindrical symmetry in most practical situations because:
the optical output of the lamps and the design of the lamp reflector surface geometry are usually such that a uniform optical flux reaches the wafer surface; PA1 an excess radiative loss at the edge of the wafer causes localized edge cooling; and PA1 the edge cooling effect usually has a cylindrical symmetry.
Thus, the wafer edge temperature can be a few degrees or a few tens of degrees (Celcius) cooler than the center during the thermal cycling process in the RTP (particularly during the constant-temperature soak or during the rapid cool-down periods of the thermal cycle).
The temperature nonuniformity may produce slip dislocation lines on the wafer (particularly near the edge) which are dislocations caused in the crystal structure of the silicon due to unequal movement of atomic planes therein from thermally induced stresses. The slip line formation problem can be particularly severe in the high-temperature regime (e.g. T&gt;900.degree. C.) since the silicon yield strength decreases with increasing temperature. The end result may be a formation of electrically active defects which degrade the circuitry and will require disposal of the entire wafer due to excessive yield losses. It is imperative to prevent formation of slip dislocations in order to make RTP more compatible with the needs of semiconductor manufacturing environments.
In one attempt to compensate for the cooler edge temperatures and prevent slip dislocations, special reflector designs have been created in order to tailor the incident optical flux on the wafer such that the photon flux increases slightly towards the edge. This arrangement provides more heat to the edges of the wafer during the steady-state segment of the wafer heating cycles. However, providing more heat to the wafer edges may also cause a transient temperature overshoot near the edges with respect to the wafer center during a rapid heat-up transient which makes the edge hotter than the center, creating the same "slip" problem. Additionally, since the heat compensation is entirely dependent upon the reflector design, providing extra heat to the edge of the wafer through a special reflector is not a general solution to the temperature nonuniformity problem since it does not allow sufficient flexibility over a wide range of temperatures for both transient and steady-state conditions.
Another attempt to prevent cooler edge temperatures and minimize slip formation has been to place a silicon ring around and in contact with the edge of the wafer. The silicon ring provides extra thermal mass to retain heat on the edge of the wafer, but again does not offer sufficient flexibility over a wide range of temperatures. Thus, the need exists for a method and apparatus to provide uniform temperature control in lamp heated, single-wafer RTP systems that is flexible over a wide range of temperatures and operates based on a real-time monitoring of the wafer temperature uniformity.